Rashi, Rashbam (citing Menachos, 96b), Targum Onkelos and Targum Yonasan: The "Misgeres" was a frame (or a rim - Rashi in Sukah, 5a) that surrounded the Shulchan ¹ , some say that it surrounded the actual table-top ² : according to others it surrounded the legs, and the table-top rested on it.

Seforno: The two crowns, which stood signified Keser Malchus, represented the two issues over which a king has control a. Justice and matters concerning the running of the state; b. Security (against the state's enemies). ¹ The Zer on the Shulchan itself represented the former, the Zer on the Misgeres, the latter. ²

Rashi: The current command is not to make a Zer Zahav, but to place it on top of the Misgeres.

Bechor Shor #1 and Hadar Zekenim #1: Because it served as a Zer for the Misgeres which was made of wood, and which the golden Zer covered.

Bechor Shor #2 and Hadar Zekenim #2: The Misgeres was made of gold, and what the Torah means is that they should make a Zer one Tefach thick to serve as a Misgaeres for the Shulchan.

R. Bachye: With reference to 25:23:2:1 - it was twelve Tefachim long, six Tefachim wide and nine Tefachim tall - since each Amah comprised six Tefachim. ¹ And it was placed lengthways in the Kodesh (close to the northern wall).